Nowadays, demands for general method for coding motion picture and its attached sound are increasing for various applications including digital storage media, or television broadcasting and communication. Motion Picture Experts Group (MPEG) in the International Electrotechnical Commission (IEC) of International Standardization Organization (ISO) has proceeded international standardization of motion picture coding method and already standardized several methods including ISO/IEC 13818-2 (MPEG2video).
Here a description is given, with reference to FIG. 9, of a conventional digital motion picture decoding apparatus for decoding the data coded by the MPEG coding method which is disclosed by Japanese Published Patent Application Number H8-32927, for example.
Reference numeral 900 designates a conventional digital motion picture decoding apparatus, numeral 911 designates a channel selecting means which takes coded video streams corresponding to plural programs as inputs and, in response to a program select signal input by a user, selects a bit stream corresponding to one of the plural programs to output the bit stream as coded picture data 901. This means is also configured to output a program change signal at the same time. The output coded picture data 901 passes through a code pattern detecting means 903 which is described later and is sequentially stored in an input buffer memory 902.
The code pattern detecting means 903 is a block which inhibits data from passing through to the input buffer memory 902 until a prescribed code pattern, I picture (intra-coded picture) of GOP (Group of pictures), is detected based on a change detection signal from the coded data change detecting means 904, and this means normally allows the data to pass through during the reproduction processing.
Reference numeral 905 designates a decoding means which makes a request for data to the input buffer memory 902, where the data obtained is subjected to decoding processing and then successively stored into a predetermined reproduced picture area in a reproduced picture memory 907.
A display picture deciding means 906 decides a reproduced picture to be displayed according to time information of each reproduced picture and the information about the data stored in the reproduced picture memory 907 obtained from a time information managing means 908 and a reproduced picture area managing means 909, outputting the picture as a reproduced picture data 910 for reproduction and display.
Next, a description is given of the operation for a transition from normal reproduction mode to special reproduction mode in which reproduction speed is changed to realize high-speed reproduction.
In response to a request for high-speed reproduction from a user, the coded picture data 901 is transmitted to the code pattern detecting means 903 in a format which comprises only an intra-frame coded picture (I picture) extracted from the bit stream recorded in record medium, and by detecting a pattern of contiguous I pictures, it is detected that the transition to special reproduction mode has been completed. Following this, sending data through to the input buffer memory 902 is stopped and a clear signal is output from the code pattern detecting means 903 to the input buffer memory 902, whereby the data stored in the input buffer memory 902 is erased.
When the code pattern detecting means 903 detects normal pattern again, cancel of the special reproduction mode is recognized and a blue-back processing which is described later is canceled, whereby sending data to the input buffer memory 902 is resumed.
Next, a description is given of the operation in the above-described configuration when the coded data in the process of reproduction becomes partially discontinuous due to change of program.
After detecting based on a program change detection signal that change of the coded data to be input has been performed, the coded data change detecting means 904 sends a clear signal to the input buffer memory 902 to clear all the data stored in the memory 902, while sending a data passing inhibit signal to the code pattern detecting means 903 in order to inhibit data from passing through to the input buffer memory 902 until a start code (I picture) of an intra-frame coded picture is detected by the code pattern detecting means 903.
At this time, blank pictures (Bu pictures) are inserted following a bidirectional predictive-coded picture B3 (B picture), resulting in blue-back processing which provides a screen turned to blue.
Then, when the code pattern detecting means 903 detects the start code (I picture) of the intra-frame coded picture from the coded picture data after the change, passing data through to the input buffer memory 902 is permitted again and the blue-back processing is canceled, whereby reproduction operation is started again.
The decoding apparatus is configured to prevent reference picture error caused by connecting data arrays of different types at the change of coded data and avoid decoding the data left in the buffer at the change of the coded picture data, by performing the above-described processing.
The prior art digital motion picture decoding apparatus and the digital motion picture decoding method have the above-described construction which prevents reference picture error caused by connecting different types of data arrays at the change of coded data, but both the conventional decoding apparatus and the method have a problem that when the coded data is changed due to the change of program or other causes, blue-back screen is maintained during a prescribed period of time, making a viewer feel unconformable.
Moreover, in the case that change of coded data is not performed on a single piece of picture data basis and the coded data to be input is changed in the process of decoding a single piece of picture data by means of the decoding means 905, the operation of changing program in the middle of decoding processing as shown in FIG. 8(a) allows a situation as shown in FIG. 8(b) which is equal to the situation caused by inputting a data array to which a different type of data array is connected, resulting in decoding error and the considerably confused display resulted from error picture.
Furthermore, as shown in FIG. 5, since the structure of the data coded by a method employing bidirectional coding such as MPEG coding has an order different from both the order in which the picture data of the coded data is placed and the display order for reproducing pictures, forward predictive coded picture (P pictures), which is to be displayed following a sequence of bidirectional predictive coded pictures (B pictures), is decoded before decoding of the bidirectional predictive coded pictures (B pictures), and the resultant decoded data is stored in the reproduced picture memory 907. Therefore, as shown in FIG. 6), in the configuration only capable of clearing the data in the input buffer memory at the last stage, when the decoding processing has been already completed at the change of the coded picture data to be input and reproduced picture data which is not displayed yet (P6) is stored in the reproduced picture memory because the display time is not reached, that reproduced picture data is certainly displayed when the display time based on time information of the reproduced picture is reached. Therefore, with a long interval between each display time of reproduced pictures, an extended period of time is required before displaying the reproduced picture data which is unnecessary (P6) and it takes much time to change the coded picture data in the process of reproduction, whereby the blue-back screen is displayed for an extended period of time and makes a viewer feel uncomfortable.